1. Field of Invention
This invention relates to interface apparatus between data terminal equipment (DTE), such as keyboards, printers, I/O devices and the like, and data communication equipment (DCE), such as modulator/demodulators (modems). More specifically, this application relates to apparatus for interconnecting data terminal equipment with data communication equipment which interface according to the provisions of both EIA standard RS-232C and EIA Standard RS-449 employing balanced voltage digital interface circuits according to EIA electrical Standard RS-422.
The Electronic Industries Association (EIA) has provided engineering standards designed to facilitate interchangeability of equipment for transmission of digital data. For many years, the industry standard of interface between DTE and DCE has been the provisions of EIA standard RS-232C. Standard RS-232C specifies both electrical characteristics and physical characteristics, including protocol, for digital data interchange. RS-232C specifies a 25-position basic mechanical connection between a DTE and DEC. In addition, RS-232C specifies the transmission and reception of data by means of unbalanced voltage digital interface generators and receivers of defined electrical characteristics.
It was the intent of EIA Standard RS-232C that it be applicable to data transmission speeds up to about 20,000 symbols per second over relatively short cable lengths between a single modulator/demodulator (modem) and a single terminal device. The characteristics of the RS-232C electrical interface are not suitable for handling data and timing signals in the megabit range due, for example, to increased crosstalk between the normally unbalanced circuits and due to the reflections caused by the typical lack of matched impedance terminations in the simple wire conduit interchange circuits.
EIA Standard RS-449 specifies functional characteristics through a 37-position mechanical interconnection capable of providing multiple-point (multi-drop) interconnection through a single cable. The RS-449 standard is designated to employ two types of electrical characteristics, namely, RS-422, specifying balanced differential voltage digital interface circuits, and RS-423, specifying an unbalanced voltage digital interface circuits. These interface standards may be referred to herein as RS-449/422 and RS-449/423.
In order to obtain maximum data rates and maximum distance of data transmission without undue distortion, the use of a balanced data communication line has been recommended. One of the reasons for this recommendation is the characteristic of common mode noise rejection found only in a balanced line. As a consequence, an interface using 24-gauge twisted pair cable interconnection circuit according to RS-422 electrical standards is capable of transmitting data through cable lengths of 4,000 feet at rates up to 100,000 symbols per second, and further is capable of transmitting data over cables of 50 feet long at rates up to about 7.5 million symbols per second. In addition, both RS-422 and RS-423 are intended to support multiple-terminal (multipoint) operation of DEC and DTE.
Since the recommended adoption of the RS-449 37-position interface for DTE and DCE in November 1977, there has been considerable resistance to the adoption of the 37-pin connection standard. It has been found, for example, that most of the 37 lines are not needed by the DTE. Moreover, it has been found that the cost of cabling a full 37-position line may often exceed the cost of the equipment itself. For example, a cable 4,000 feet long may cost as much as four times the cost of the equipment to which it is connected, at current material prices. Hence, data communication equipment manufacturers especially have been reluctant to incorporate a 37-pin connector RS-449 interface into modem equipment used to communicate with communication lines, such as telephone lines. Nevertheless, the RS-449/422 electrical standard represents a substantial advance in the rate capacity and versatility of interface circuits. What is needed therefore is data terminal equipment and data communication equipment interface circuitry which is capable of communication with both RS-232C standard interface circuitry and a subset of RS-449-type interface circuitry taking full advantage of the RS-422 electrical standard to permit successful integration of advanced high-speed equipment into communication systems in a manner which is compatible with industry-wide standards.
2. Description of the Prior Art
Industrial Electronics Bulletin No. 12 dated November 1977 entitled, "Application Notes on Interconnection Between Interface Circuits Using RS-449 and RS-232-C" published by the Electronic Industries Association suggests the current accepted method and apparatus for interconnecting RS-232C-type DTE to RS-449-type DCE as well as for interconnecting RS-449-type DTE to RS-232C-type DEC. This Bulletin is intended to specify all necessary electrical, function and mechanical consideration necessary for satisfactory operation between RS-232C-type equipment and RS-449-type equipment. Basically, the Bulletin suggests that RS-449-type equipment be derated to operate in the RS-232C environment using RS-423 electrical characteristics adapted to the particular application. Since RS-423 electrical characteristics specify unbalanced circuits, near equivalency can be obtained for interoperability by placing an attenuator pad between an RS-232C generator and RS-422 receiver. RS-422-type receivers are converted to unbalanced connectors by grounding one side of the receiver input.
The Bulletin further suggests the use of external or in-cable 25- to 37-pin adapters for interconnection configurations. Moreover, the Bulletin suggests the use of different connectors for RS-449 as an advantage for preventing inadvertent connection of RS-449 equipment to RS-232C equipment. The larger 37-position connector as well as a supplemental 9-position connector specified in the RS-449 standard have nevertheless found little acceptance since it adversely affects both the price and the reliability of the host equipment. What is needed is a connection which will enable the interoperability of both RS-232C-type equipment and RS-449/422-type equipment in a smaller, i.e., 25-position connector. In this manner, RS-232C DCE can be connected to RS-449 or RS-232C-type DTE, and the inconvenient external adapter can be eliminated. The supplier of DTE may therefore provide both the terminal equipment and the cabling for connection with the DTE provided by a modem manufacturer or supplier. The DCE supplier may therefore rest assured that the equipment can be made to neet the needs of both existing equipment users and future equipment users.